The Effect of Hydrogen on Low-Cycle Fatigue Behaviour of a High-Manganese Steel: Make a TWIP Steel Show TRIP Behaviour

We systematically investigated the effect of hydrogen on the low-cycle fatigue (LCF) behaviour of a Fe-28Mn-0.3C (wt.%) twinning-induced plasticity (TWIP) steel by comparing the fatigue microstructure of samples with and without hydrogen pre-charging using electron channelling contrast imaging (ECCI) and electron backscatter diffraction (EBSD). The results reveal that a complex interplay of several hydrogen-microstructure interaction mechanisms is involved in the observed rapid hydrogen embrittlement of the studied TWIP steel under LCF. The associated mechanisms for each step are clarified in detail; first, hydrogen assists the nucleation of stacking faults and deformation-induced epsilon martensite from grain boundaries (GBs) by reduction of local stacking fault energy (Suzuki effect) and stabilization of the hcp e-phase. The evolution of fatigue dislocation patterns is strongly retarded in the presence of hydrogen, which could be ascribed to the HELP mechanism or/and the stress and strain-shielding effect of e-martensite plates. The rapid formation of e-martensite leads to stronger cyclic hardening. The impingement of e-martensite plates at GBs leads to high local stress concentrations. At the same time, the martensite-generating dislocations provide an efficient way for hydrogen transport from the grain interior into the boundary which leads to strong local embrittlement at these places. As a consequence, fatigue cracks are initiated even after as few as 5 cycles at a strain amplitude of around 1 %, which can be ascribed to hydrogen-enhanced decohesion (HEDE). Among different GB types, annealing twin boundaries and low angle grain boundaries show highest immunity to fatigue cracking, which is in contrast to the frequent observation that twin boundaries are weak against HEDE in low-SFE materials.